linux-kernel - Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync

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Date:   Thu, 29 Nov 2018 19:35:50 +0100
From:   Christoph Hellwig <hch@....de>
To:     Brian Starkey <Brian.Starkey@....com>
Cc:     Christoph Hellwig <hch@....de>, "Clark, Rob" <robdclark@...il.com>,
        Dave Airlie <airlied@...ux.ie>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Tomasz Figa <tfiga@...omium.org>,
        Sean Paul <seanpaul@...omium.org>,
        "vivek.gautam@...eaurora.org" <vivek.gautam@...eaurora.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        Robin Murphy <Robin.Murphy@....com>, nd <nd@....com>
Subject: Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*

On Thu, Nov 29, 2018 at 05:33:03PM +0000, Brian Starkey wrote:
> This sounds very useful for ion, to avoid CPU cache maintenance as
> long as the buffer stays in device-land.
> 
> One question though: How would you determine "the last user to unmap"
> to know when to do the final "make visible to CPU" step?

I'd assume the user of the DMA API keeps track of it.  E.g. dmabuf
would be a pretty natural layer to implement common logic for this.